A typical prior art electronic scale comprises a load cell, which provides an analog voltage representative of weight on the scale, an analog-to-digital convertor ("ADC") for converting the analog voltage to a digital form, and a microprocessor with associated interface circuitry for transforming the digital output from the ADC into appropriate units of weight. The system electronics typically includes amplification circuitry for transforming the raw voltage from the load cell into a suitable range for input to the ADC. One type of commercially available monolithic ADC provides a 16-bit digital output (15 bits plus sign bit) representative of an analog input voltage received at a set of input terminals. The permissible range of input voltages may be defined by an external reference, with .+-.2-4 volts being typical.
Clearly, a key factor in the performance of the scale is the ability of the ADC to provide a digital representation of the load cell voltage with sufficient resolution. Consider the example of a 100-pound scale where the ADC has a .+-.3.2-volt input range. Assuming the load cell voltage is appropriately offset and amplified to provide the full .+-.3.2-volt swing for the 0-100 pound range of weights, the ADC output will range over .+-.32K counts, providing approximately 41 counts per ounce. Unfortunately, the ADC output is not stable but is characterized by significant drift (perhaps tens of counts over a period of minutes). Although the drift is slow enough that it can be tracked and compensated, the weight is not accurate to a single count. Accordingly, the actual resolution of the scale is coarser than the 1/41-ounce theoretical maximum. Indeed, with tracking instabilities of one or two counts, conservative design would allow four counts (approximately 1/10 ounce) as the smallest weight subdivision. This is certainly acceptable for certain applications, but not others. Greater sensitivity, if needed, may be achieved by configuring the scale for a lower weight range (a 25-pound maximum would provide the 1/40-ounce resolution) or by using an ADC with more bits.